The present invention relates generally to fuel cells and relates more particularly to direct organic fuel cells.
Fuel cells are electrochemical cells in which a free energy change resulting from a fuel oxidation reaction is converted into electrical energy. Because of their comparatively high inherent efficiencies and comparatively low emissions, fuel cells are presently receiving considerable attention as a possible alternative to the combustion of nonrenewable fossil fuels in a variety of applications.
A typical fuel cell comprises a fuel electrode (i.e, anode) and an oxidant electrode (i.e., cathode), the two electrodes being separated by an ion-conducting electrolyte. The electrodes are connected electrically to a load, such as an electronic circuit, by an external circuit conductor. Oxidation of the fuel at the anode produces electrons that flow through the external circuit to the cathode producing an electric current. The electrons react with an oxidant at the cathode. In theory, any substance capable of chemical oxidation that can be supplied continuously to the anode can serve as the fuel for the fuel cell, and any material that can be reduced at a sufficient rate at the cathode can serve as the oxidant for the fuel cell.
In one well-known type of fuel cell, sometimes referred to as a hydrogen fuel cell, gaseous hydrogen serves as the fuel, and gaseous oxygen, which is typically supplied from the air, serves as the oxidant. The electrodes in a hydrogen fuel cell are typically porous to permit the gas-electrolyte junction to be as great as possible. At the anode, incoming hydrogen gas ionizes to produce hydrogen ions and electrons. Since the electrolyte is a non-electronic conductor, the electrons flow away from the anode via the external circuit, producing an electric current. At the cathode, oxygen gas reacts with the hydrogen ions migrating through the electrolyte and the incoming electrons from the external circuit to produce water as a byproduct. The overall reaction that takes place in the fuel cell is the sum of the anode and cathode reactions, with part of the free energy of reaction being released directly as electrical energy and with another part of the free energy being released as heat at the fuel cell.
It can be seen that as long as oxygen and hydrogen are fed to a hydrogen fuel cell, the flow of electric current will be sustained by electronic flow in the external circuit and ionic flow in the electrolyte. Oxygen, which is naturally abundant in air, can easily be continuously provided to the fuel cell. Hydrogen, however, is not so readily available and specific measures must be taken to ensure its provision to the fuel cell. One such measure for providing hydrogen to the fuel cell involves storing a supply of hydrogen gas and dispensing the hydrogen gas from the stored supply to the fuel cell as needed. Another such measure involves storing a supply of an organic fuel, such as methanol, and then reforming or processing the organic fuel into hydrogen gas, which is then made available to the fuel cell. However, as can readily be appreciated, the reforming or processing of the organic fuel into hydrogen gas requires special equipment (adding weight and size to the system) and itself requires the expenditure of energy.
Accordingly, in another well-known type of fuel cell, sometimes referred to as a direct organic fuel cell, an organic fuel is itself oxidized at the anode. Examples of such organic fuels include methanol, ethanol, propanol, isopropanol, trimethoxymethane, dimethoxymethane, dimethyl ether, trioxane, formaldehyde, and formic acid. Typically, the electrolyte in such a fuel cell is a solid polymer electrolyte or proton exchange membrane (PEM). (Because of the need for water in PEM fuel cells, the operating temperature for such fuel cells is limited to approximately 130xc2x0 C.) In operation, the organic fuel is delivered to the anode in the form of a fuel/water mixture, and airborne oxygen is delivered to the cathode. (Oxidants other than oxygen, such as hydrogen peroxide, may also be used.) Protons are formed by oxidation of the organic fuel at the anode and pass through the proton exchange membrane to the cathode. Electrons produced at the anode in the oxidation reaction flow in the external circuit to the cathode, driven by the difference in electric potential between the anode and the cathode and, therefore, can do useful work. A summary of the electrochemical reactions occurring in a direct organic fuel cell (with methanol illustratively shown as the organic fuel) are as follows:
Anode: CH3OH+H2Oxe2x86x92CO2+6H++6exe2x88x92xe2x80x83xe2x80x83(1)
Cathode: 1.5O2+6H++6exe2x88x92xe2x86x923H2Oxe2x80x83xe2x80x83(2)
Overall: CH3OH+1.5O2xe2x86x92CO2+2H2Oxe2x80x83xe2x80x83(3)
At present, there are two different types of systems that incorporate direct organic fuel cells, namely, liquid feed systems and vapor feed systems. Examples of liquid feed systems are disclosed in the following U.S. patents, all of which are incorporated herein by reference: U.S. Pat. No. 5,992,008, inventor Kindler, issued Nov. 30, 1999; U.S. Pat. No. 5,945,231, inventor Narayanan et al., issued Aug. 31, 1999; U.S. Pat. No. 5,599,638, inventors Surampudi et al., issued Feb. 4, 1997; and U.S. Pat. No. 5,523,177, inventors Kosek et al., issued Jun. 4, 1996.
In a typical liquid feed system, a dilute aqueous solution of the organic fuel (i.e., approximately 3-5 wt % or 0.5-1.5 M organic fuel) is delivered to the fuel cell anode whereupon said aqueous solution diffuses to the active catalytic sites of the anode, and the fuel therein is oxidized. The liquid feed system is typically operated at 60xc2x0 C.-90xc2x0 C. although operation at higher temperatures is possible by pressurizing the anode and the fuel supply system. (For operation at temperatures greater than 100xc2x0 C., cathode pressurization is additionally required.)
As can readily be appreciated, it would be desirable to increase fuel cell performance in a liquid feed system by using a more concentrated solution of the organic fuel than the approximately 3-5 wt % solution described above. Unfortunately, however, the proton exchange membrane typically used in a liquid feed system is rather permeable to the organic fuel. As a result, a substantial portion of the organic fuel delivered to the anode has a tendency to permeate through the proton exchange membrane, instead of being oxidized at the anode. Moreover, much of the fuel that transits the proton exchange membrane is chemically reacted at the cathode and, therefore, cannot be collected and recirculated to the anode. This type of fuel loss, which can total as much as 50% of the fuel, is referred to in the art as crossover. In addition, this problem of cross-over is exacerbated if the concentration of organic fuel in the aqueous solution is increased beyond the approximately 3-5 wt % described above since the permeability of the proton exchange membrane increases exponentially as the organic fuel concentration increases.
Consequently, because the concentration of organic fuel in the aqueous solution must remain relatively low to minimize cross-over, large quantities of water must be made available for diluting the organic fuel to appropriate levels. However, as can be appreciated, the required quantities of water can be heavy and space-consuming and can pose a problem to the portability of the system. Moreover, equipment for mixing the water and the organic fuel in the appropriate amounts, for re-circulating water generated at the cathode and for monitoring the concentration of the organic fuel in the aqueous solution is often needed as well.
Another complication resulting from the high concentration of water present in the aqueous solution is that a considerable amount of water delivered to the anode also permeates through the proton exchange membrane to the cathode. This excess water arriving at the cathode limits the accessibility of the cathode to gaseous oxygen, which must be reduced at the cathode to complement the oxidation of the fuel at the anode. This phenomenon of the permeating water accumulating at the cathode and, thereby, limiting the accessibility of the cathode to gaseous oxygen is referred to in the art as flooding. As can readily be appreciated, flooding adversely affects fuel cell performance.
In a typical vapor feed system, the aqueous solution of organic fuel and water is vaporized prior to entering the fuel cell and is then fed, in vapor form, to the anode. Because the proton exchange membrane is less permeable to the fuel/water mixture in vapor form than it is to the fuel/water mixture in liquid form, the above-described problems of cross-over and flooding are less pronounced in a vapor feed system. As a result, fuel cell performance and fuel efficiency are typically greater in a vapor feed system than in a liquid feed system. Moreover, due to the decreased permeability of the membrane to the fuel/water mixture in its vapor form, a higher concentration of the organic fuel may be employed in a vapor feed system.
However, some of the advantages of a typical vapor feed system are that the system must be operated at above 100xc2x0 C. in order to prevent condensation of the fuel/water mixture at the anode. In addition, the fuel/water mixture must be vaporized prior to entering the fuel cell. As can be appreciated, the foregoing conditions require the use of specialized equipment that is space-consuming and that requires the expenditure of energy for its own operation. Moreover, due to the amount of heat that is generated as an unwanted byproduct in the fuel cell, a vapor feed system must also include a cooling assembly, typically in the form of coolant plates and a circulating coolant, to keep the fuel cell from getting too hot. Such a cooling assembly can add considerable weight and volume to the system, especially if a multi-cell stack is used, since one cooling plate is needed for every 2-5 active cells. (By contrast, in a liquid feed system, the aqueous solution, in addition to containing the fuel, also serves as a coolant for the system.)
It is an object of the present invention to provide a novel direct organic fuel cell.
It is another object of the present invention to provide a novel direct organic fuel cell that. overcomes at least some of the drawbacks discussed above in connection with existing direct organic fuel cells.
Therefore, according to one aspect of the invention, there is provided a fuel cell suitable for use as a direct organic fuel cell, said fuel cell comprising (a) a membrane electrode assembly, said membrane electrode assembly comprising (i) a proton exchange membrane, said proton exchange membrane having a front face and a rear face, (ii) an anode, said anode coupled to said front face of said proton exchange membrane, and (iii) a cathode, said cathode coupled to said rear face of said proton exchange membrane; (b) a vapor diffusion chamber, said vapor diffusion chamber being positioned in front of said anode; (c) a vapor transport member positioned in front of said vapor diffusion chamber, said vapor transport member being substantially impermeable to an organic fuel and water mixture in a liquid phase but being permeable to said organic fuel and water mixture in a vapor phase; and (d) means for electrically interconnecting said anode and said cathode through an external load.
More specifically, in a first preferred embodiment, the aforementioned fuel cell comprises (a) a membrane electrode assembly, said membrane electrode assembly comprising (i) a proton exchange membrane, said proton exchange membrane having a front face and a rear face, (ii) an anode, said anode coupled to said front face of said proton exchange membrane and preferably including a platinum-ruthenium electrocatalytic film and (iii) a cathode, said cathode coupled to said rear face of said proton exchange membrane and preferably including a platinum electrocatalytic film. An electrically-conductive, vapor-permeable member, which may be, for example, a metal screen package, is positioned in front of and in contact with said anode, said electrically-conductive, vapor-permeable member defining a vapor diffusion chamber and serving as the negative terminal of the fuel cell. A vapor transport member, which may be, for example, a perfluorosulfonic acid membrane, is positioned in front of and in contact with said electrically-conductive, vapor-permeable member, said vapor transport member being substantially impermeable to an organic fuel and water mixture in a liquid phase but being permeable to said organic fuel and water mixture in a vapor phase. A support that is porous to liquid, which support may be, for example, carbon fiber paper, is positioned in front of and in contact with said vapor transport member, said vapor transport member extending beyond the periphery of said support. A fuel distribution plate is positioned in front of and in contact with both the support and the periphery of the vapor transport member, the support being seated upon a recessed shelf formed within a cavity of the fuel distribution plate, the cavity facing towards said vapor transport member and being adapted to hold a quantity of a liquid fuel. The fuel distribution plate is additionally shaped to include an array of pillars or like supportive members dispersed throughout the cavity, said supportive members serving both to distribute the fuel throughout the cavity and to provide structural support to the support and the vapor transport member. An oxidant distribution plate, which is electrically conductive and is positioned behind and in contact with said membrane electrode assembly, has a cavity facing said cathode, said cavity being adapted to hold a quantity of an oxidant. The oxidant distribution plate is additionally shaped to include an array of electrically-conductive pillars or like supportive members dispersed throughout the cavity, said electrically-conductive supportive members serving to provide support to the membrane electrode assembly, to distribute the oxidant throughout the cavity and to provide electrical contact between the cathode and the remainder of the plate. A first endplate is positioned in front of and in contact with said fuel distribution plate and a second endplate is positioned behind and in contact with said oxidant distribution plate, said first and second endplates serving to apply axially compressive force to the components located therebetween. In addition, said second endplate serves as a positive terminal and is electrically connected, by way of said oxidant distribution plate, to said cathode.
In a second preferred embodiment, there is provided a co-planar direct organic fuel cell assembly, said co-planar direct organic fuel cell assembly comprising a plurality of co-planar fuel cells and means for coupling together adjacent fuel cells. Each of said co-planar fuel cells comprises a membrane electrode assembly, said membrane electrode assembly comprising (a) a proton exchange membrane, said proton exchange membrane having a top face and a bottom face, (b) an anode, said anode coupled to said bottom face of said proton exchange membrane, and (c) a cathode, said cathode coupled to said top face of said proton exchange membrane. An anode current collector, which is electrically-conductive and vapor-permeable, is positioned below and in contact with said anode, said anode current collector defining a vapor diffusion chamber. A vapor transport assembly, comprising a vapor transport member sandwiched between a pair of porous supports, is positioned below and in contact with said anode current collector, said vapor transport member being substantially impermeable to an organic fuel and water mixture in a liquid phase but being permeable to said organic fuel and water mixture in a vapor phase. An anode basin is positioned below and in contact with said vapor transport assembly, said anode basin having a cavity facing towards said vapor transport member, said cavity being adapted to hold a quantity of a liquid fuel. A cathode current collector is positioned over and in contact with said cathode, said cathode current collector being electrically coupled to said anode current collector through an external load or adapted to be electrically coupled to the anode current collector of an adjacent fuel cell.
The present invention is also directed to a system including one or more direct organic fuel cells of the present invention and means for supplying said one or more direct organic fuel cells with said organic fuel/water mixture.
The present invention is further directed to a method of generating electricity using the direct organic fuel cell of the present invention, as well as to a method of generating hydrogen-containing species, including hydrogen gas, using the direct organic fuel cell of the present invention.
As can be seen, a system comprising the direct organic fuel cell of the present invention possesses certain advantages of the conventional liquid feed system, such as relative system simplicity, while, at the same time, possessing certain advantages of the conventional vapor feed system, such as improved performance and reduced fuel cross-over. In short, such a system is characterized by high performance, high fuel efficiency, high gravimetric and volumetric power densities and ease of operation.
As can readily be appreciated, the direct organic fuel cell of the present invention can be operated conventionally (in the case of a direct methanol fuel cell, to generate carbon dioxide, water and electricity using methanol and gaseous oxygen from air or other oxygen-containing sources) or can be used, in a first alternative application, to generate gaseous hydrogen by additionally supplying electricity to the cell and preventing oxygen from reaching the cathode or, in a second alternative application, to generate a hydrogen-containing species at the cathode by additionally supplying electricity to the cell while providing a reducible species to the cathode. For purposes of the present specification and claims, all references herein to the direct organic fuel cell of the present invention are intended to encompass said fuel cell both in its conventional operation and in the above-described alternative operations to produce hydrogen or a hydrogen-containing species unless otherwise specified or apparent from context.
For purposes of the present specification and claims, it is to be understood that certain terms used herein, such as xe2x80x9con,xe2x80x9d xe2x80x9cover,xe2x80x9d and xe2x80x9cin front of,xe2x80x9d when used to denote the relative positions of two or more components of a fuel cell, are used to denote such relative positions in a particular orientation and that, in a different orientation, the relationship of said components may be reversed or otherwise altered.
Additional objects, as well as features and advantages, of the present invention will be set forth inpart in the description which follows, and in part will be obvious from the description or may be learned by practice of the invention. In the description, reference is made to the accompanying drawings which form a part thereof and in which is shown by way of illustration various embodiments for practicing the invention. The embodiments will be described in sufficient detail to enable those skilled in the art to practice the invention, and it is to be understood that other embodiments may be utilized and that structural changes may be made without departing from the scope of the invention. The following detailed description is, therefore, not to be taken in a limiting sense, and the scope of the present invention is best defined by the appended claims.